The Platelet Monitor System has been designed to assess routinely prepared platelet concentrates (P.C.) during storage. The system is based upon observations of changes in light transmission induced by platelets in intact packs. It is practical, provides constant gentle agitation for P.C., and non-invasively give continuous semi-quantitative evidence of the probable numbers of discoid platelets within each pack. The information yielded correlates both with established in vitro parameters of platelet behaviour, and with in vivo survival of platelets in normal healthy volunteers. The device safely provides the Blood Banker with the means to achieve on-going assessment of methods and improvements--or failings--in P.C. production. To the clinician it offers information throughout and even beyond present arbitrary storage limits, about which P.C. are likely to be of least or greatest value to a thrombocytopenic patient.